Vehicles are typically equipped with an antenna for receiving radio signals. One example of such an antenna is a mast antenna, which extends from the exterior body of the vehicle. However, the mast antenna is generally susceptible to interfering with the desired styling of the vehicle, being damaged when the vehicle passes under a low clearance object, acts of vandalism, accident with another vehicle or object, and has limitations in the terms of its reception quality.
An alternative to the mast antenna is placing the antenna within the vehicle's glass, such as a windshield. Whether the single antenna is a mast antenna, an in-glass antenna, or other type of antenna, a single antenna typically has inherent limitations, such as fading and multipath signal interference resulting from an obstruction, which can be caused by the presence of a building, a mountain, another vehicle, or the like. Further, the in-glass antennas are typically more susceptible to fading and multipath signal interference due to their gain, their directivity, and their polarization properties. There have been several techniques developed using multiple antennas for receiving radio signals to reduce the affects of such fading and interference. These techniques include scanning/selection or switching diversity, equal-gain combining, and maximal-ratio combining. The scanning/selection or switching diversity technique is one that operates on the premise that if one antenna disposed on the vehicle is receiving a poor signal, another antenna spaced from the first antenna may be receiving a better signal. Thus, only one antenna is used for receiving the signal at any particular point in time. The system either compares the signals that are being received by the system's antennas to ascertain which antenna is receiving the better quality signal, or the system evaluates the signal being received by a single antenna to determine a quality of the signal and simply switches to another antenna if the current signal is designated as unacceptable. However, the switching transients caused by switching between antennas can be audible under some circumstances, and since only one antenna is typically used at any point in time, the system may provide only marginal improvement during fringe reception when compared to single antenna systems.
The equal-gain combining technique combines signals received by the antennas in an antenna array by correcting for the phase differences between antennas, then adding the signals pictorially. No adjustments are made to the signals for any difference in the gains of the input signals because only the phases of the input signals are adjusted for alignment in an equal-gain system. However, it is possible that the signal-to-noise ratio may be less than optimal. For example, if two inputs are combined, and one of those inputs contains mostly noise, the combined signal is likely to be of lower quality than the single non-corrected signal. In such a situation, it would have been ideal to use only the signal from the antenna that was not mostly noise.
Another technique is the maximal-ratio combining technique. In the maximal-ratio combining technique, the input signals are adjusted according to the detected phase thereof, the magnitudes of the input signals are adjusted according to the detected phase thereof, and the magnitudes of the input signals are adjusted to yield the maximum signal-to-noise ratio. Thus, a signal that is corrupted with noise does not degrade the overall performance of the system. However, the maximal-ratio combining technique is generally very complex, typically, due to the hardware having multiple receivers plus the combined algorithm for combining the multiple signals. Additionally, the cost of implementing such a system can be prohibitive in some environments.
In the early 1960s, an equal-gain combining technique was developed that permitted phase alignment at the radio frequency (RF) Lewin, “Diversity Reception and Automatic Phase Correction” (Proc. of IEEE, Paper No. 3584E, Vol. 9, Part B., No. 46, pp. 295-304, July 1962). In Lewin, a phase changer was disclosed for use in an adaptive system. The phase changer both sensed and corrected the phase of the signal. Specifically, phase perturbation is introduced, and the resulting amplitude modulation is detected. Based on the work of Lewin, others developed similar techniques for amplitude modulated (AM) receivers (Parsons et al., “Space Diversity Reception for VHF Mobile Radio,” Electronic Letters, Vol. 7, No. 22, pp. 655-56, Nov. 4, 1971). For frequency modulated (FM) receivers, a related technique was developed (Parsons et al., “Self-Phasing Aerial Array for FM Communication Links,” Electronic Letters, Vol. 7, No. 13, pp. 380-81, Jul. 1, 1971). In the system described in Parsons, amplitude perturbation is introduced, which results in phase modulated components of the sum signal, which are proportional to the relative phases of the input signals. This phase perturbation is then detected and used in a feedback loop to control phase shifters and bring the input signals into phase alignment. The perturbation frequency must be outside the modulation bandwidth to avoid interference with a legitimate FM signal.
Further, the above systems generally lack gain-control of the antenna signals to optimize the signal-to-noise ratio of the output. The signal-to-noise ratio is a comparison of the power of the signal to the power of the noise. By not controlling the gain of the system, the power of the output of the system can be at an undesirable proportion to the power of the input of the system, which can result in an undesirable signal-to-noise ratio.
Therefore, it is desirable to develop a stereo receiving system and method that aligns the phases of the RF signals received by the multiple antennas and includes a gain-control loop for optimizing the signal-to-noise ratio.